There is a need for a charge indicator that can be used on common nickel metal hydride cylindrical rechargeable batteries, also known as “NIMH cells.” Typical sizes and styles are known as AAA, AA, C, D and 9-volt style package. These batteries are typically manufactured in Nickel Metal Hydride (Nimh) and Nickel Cadmium (Nicad). These rechargeable batteries can replace their counterparts being known as zinc and alkaline and can be recharged many times over, typically one thousand times, thus saving the user a considerable amount of expense for battery power over the life of these rechargeable types.
Utilizing these types of batteries presents the problem of keeping track of the charged batteries, and distinguishing them from discharged batteries. Prior art approaches include battery holder packs that can be marked charged and discharged, wherein a user needs to keep the batteries in these holders to identify their state. There are patents (such as U.S. Pat. No. 6,483,275) and designs that have the batteries themselves indicate their status of charge, a technique that requires circuits onboard and the like.
There is a growing use of battery power in the world for equipment like digital cameras that use a great deal of power and also need the portability of a small powerful universal battery. The market is growing very quickly. Battery manufacturers, as well as battery charger manufactures, have realized this and it would appear that this need for vast amounts of renewable power will continue for some time.
There is a need to identify a battery's state of charge so as not to place a discharged battery in with freshly charged batteries. This is also not wise due to discharging among themselves to level out the charge state of the group being used in the equipment. What is needed is a way to easily mark each individual battery before and after use in the equipment. It is really not necessary to know the exact state of the battery charge in most cases. The equipment in use typically has built in charge indicators, such as digital cameras to identify the level of the batteries. A typical user only cares to know that the batteries have been fully charged and that they are discharged after removing them from the equipment in use. This is why each battery needs to have an indicator. Typical charge indicators look at voltage levels, but NIMH batteries have a very distinctive discharge curve that keeps the battery at a given voltage level just before running out of power in an abrupt manner. Due to the sudden power drop-off characteristic of NIMH and Nicad batteries, prior art self-testing systems incorporated within or onto the battery cannot reliably indicate to a user whether the battery has a full usable charge.